Elements may combine through any one of the following ways to form stable compounds:
By transfer of electrons
When the chemical bond occurs by the complete transfer of electron(s) from the atom(s) of one element to the atom or atoms of another element.The bond formed is called Ionic(electrovalent) bond.
By sharing of electrons
1)When the shared electrons are contributed by the two combining atoms equally,the bond formed is called Covalent bond.
2)When these electrons are contributed entirely by one of the atoms but shared by both,the bond formed is known as Co-ordinate bond.